Code Division Multiple Access (CDMA) has emerged as a key technology for communication services. CDMA intentionally uses a transmission bandwidth that is usually several orders of magnitude larger than the minimum required signal or information bandwidth, e.g., the bandwidth implied by the basic message symbol rate in a communication system. The power of each user's signal is spread over a wide bandwidth. This results in low power spectral density and thereby the interference to another narrow band signal occupying the same frequency range is reduced. It also makes the presence of the signal less detectable to an invader.
A digital signal from each user in a CDMA system is modulated with a pseudo-noise (PN) binary sequence that is unique to that particular user. This modulation causes the spreading over the wide bandwidth. Each PN sequence appears random to a naive observer but can be reproduced in a deterministic manner by an intended receiver. Any two PN sequences are made substantially orthogonal to each other but some degree of cross correlation still exists. The mutual interference in the same frequency range of multiple users is greatly reduced in a CDMA system. This orthogonality allows multiple access within the same frequency spectrum and makes CDMA systems less vulnerable to intentional or unintentional interference. However, the cross correlation becomes limiting the system capacity as the number of users increases.
Detection of a CDMA system usually involves cross correlation with a locally-generated version of the PN sequence. A desired user signal is generally detected by cross correlating with the exact same PN sequence that is uniquely assigned to that particular user by the system. Signals from other users contribute a small amount of wideband noise due to non-zero cross correlation value produced by the cross correlation operation.
The ratio of the transmission bandwidth over the information bandwidth indicates the degree of the spectrum spreading of a CDMA system. This ratio is often referred as the processing gain. Many beneficial properties of CDMA relate to the processing gain.
FIG. 1 shows a simplified CDMA system 100. User message symbols (e.g., encoded data bits) are modulated with a carrier frequency and spreaded by PN sequence waveforms prior to transmission. A receiver system 130 process the received signals by filtering and correlation operation to recover the embedded user message symbols. A CDMA detector 137 is an essential part of the receiver 130.
CDMA systems have many unique properties. For example, multiple access communication by a large number of users in a common frequency range in the same and neighboring geographical areas are possible. This has particular significance in satellite and cellular communication systems. The number of users simultaneously supported by the system is usually proportional to the processing gain of the CDMA system. The resistance to interference increases with the processing gain. In addition, position location and velocity estimation can be measured with high accuracy in a CDMA system.
The application of CDMA in communication has been demonstrated in a number of systems in use including the IS-95 based CDMA cellular system disclosed in "On the Capacity of A Cellular CDMA System", by Gilhousen et al., IEEE Trans. Veh. Technol., Vol. VT-40(2), pp303-312, May, 1991, a PCS system based on IS-95, and the Globalstar LEO satellite system. The high capacity, high resolution time-of-arrival for positioning measurement, low probability of interception, and independent interference rejection makes CDMA communication systems one of the most promising candidates for multiuser communications.
The multiuser detection system is an important aspect of the CDMA technology. A portion of a simplified CDMA detection system is indicated by the signal mixers 133 and 135 and the detector 137 in FIG. 1. The CDMA detection system plays a substantially role in terms of the system capacity, processing speed, interference resistance, and noise reduction.
One significant limitation of the prior-art multiuser detectors in CDMA systems involves the cross-correlation information among users in signal detection. Many prior-art systems essentially neglect the cross correlation. This increases the symbol error rate performance and consequently reduces the capacity of the system. Specifically, the cross correlation noise is not separated from the autocorrelated signal in many prior-art detectors.
Another limitation of the prior-art systems is the slow processing speed inherent in the detectors. This is partially due to the complexity of the prior-art decoder processing techniques. For example, Verdu first disclosed the idea of the optimal multiuser detector in "Minimum Probability of Error for Asynchronous Gaussian Multiple Access Channels", IEEE Trans. Inform. Theory, Vol. IT-32(1), pp.85-96, 1986. An ensemble of K received asynchronous signals as convolutional codes was used in the proposed system, where K is the number of users in the system. The well-known Viterbi algorithm was used to decode these signals. Lupas and Verdu proved that Verdu's multiuser detector has a low bit error rate and is near-far resistant in "Near-Far Resistance of Multiuser Detectors in Asynchronous Channels", IEEE Trans. Comm, Vol.38(4), 1990. However, Verdu's scheme exhibited two major problems. First, the complexity of the system grows exponentially with the number of transmitted signals K. Even for a moderate value of K, Verdu's system is already hard to implement. Secondly, the whole period of the PN sequence is assumed to be modulated by only one symbol to reduce the computation, which makes the trellis become time invariant.
Several suboptimal multiuser detectors have been introduced after Verdu's detector, aiming at reducing the complexity of Verdu's system. For example, Xie et al. described a detector using sequential decoding in "Multiuser Signal Detection Using Sequential Decoding", IEEE Trans. Commm. Vol.38(5), pp.578-583, 1990. This system has a complexity proportional to K. Lupus and Verdu also disclosed a linear de-correlating detector with complexity proportional to K. However, both of the above systems make the same assumption as in Verdu's system, i.e., the whole period of the PN sequence is modulated by a single symbol.
In view of the above limitations, a new type of multiuser detector with high capacity and fast processing speed is needed to improve the existing CDMA systems. In particular, separation of the cross correlation noise from the signal and reduction of the processing complexity are desirable.
The present invention describes a multiuser detector with a dynamical system control at a high processing speed. Specifically, a double-sampling front-end processor is implemented in the preferred embodiment in accordance with the present invention. The detector further includes a dynamic control module for generating a system control .delta.-parameter based on the characteristics of the received signals (i.e., the number of users and the processing gain) a correlation processor for processing cross correlation information of the received signals, a plurality of approximation processors for recursively suppressing the cross correlation noise below a predetermined noise tolerance level based on the .delta.-parameter, and a decision processor for estimating the received symbols. The approximation processors form a plurality of substantially identical processing layers connected to each other in series. The correlation processor feeds information on both auto correlation and cross correlation of the input signals of multiple users to the first approximation processor in the series. In a preferred operation, received signals are processed layer by layer until the cross correlation noise is reduced below the predetermined level.
One aspect of the present invention is the linear dependence of the processing complexity with the number of the simultaneous users present in the system. This feature in combination with other unique properties of the preferred embodiment allows greater system capacity at faster processing speed in comparison with the prior-art systems.
Another aspect is the use of cross correlation information to substantially cancel the interference contributed by the cross correlation signals. This feature presents significant advantage over the prior-art system wherein the cross correlation signals is either neglected or is canceled by using repeated spreading or despreading of the signals. This use of cross correlation information increases the system capacity and reduces hardware complexity.
Still another aspect is the dynamically adjusted system control for a high-speed convergence in the multi-layer processing. The detector of the preferred embodiment controls the decorrelation processing in each processing layers by feeding the layer with a dynamic control parameter .delta.. An optimized .delta.-parameter is preferably adjusted and determined individually by the characteristics of a received signal such as the number of the simultaneous users, K, the processing gain, the PN sequence, and the user delay. Generation of the .delta.-parameter is a simple digital operation at a high speed in the present invention.
Other aspects of the present invention include additional system enhancement by optimally combining the two output signals from the double-sampling front-end processor, digital generation of the PN sequence, and a mechanism for handling decorrelation process at very small user delay times relative to the signal symbol duration.
The details of the preferred embodiment of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art.